The invention relates generally to the electrical system of an automotive vehicle and, more particularly, to control circuitry for controlling the supply of power to an electrically heatable windshield for providing deicing of said windshield.
Numerous electrical systems have been developed for deicing automotive windshields and backlites of the type which employ heating elements such as thin film metalization or a pattern of resistive heating wires embedded in the window material, which exhibit good optical transparency. A principal requirement of these systems is to supply power to the heating wires that, within safe limits of the window material, can provide a relatively rapid deicing of the window at temperatures well below freezing. This normally requires the generation of voltages substantially higher than the regulated battery voltage. Another requirement is to supply such heating power without adversely affecting the vehicle's battery charge requirements and electrical operation. A number of systems have been developed in which the standard vehicle alternator is employed to supply power to the resistive heating elements, the vehicle alternator for a short period being disconnected from the standard automotive circuit including the battery and load and connected to the resistive heating elements for the purpose of deicing. During this period the alternator can supply voltages substantially higher than the regulated battery voltage, while the battery supplies the normal automotive load. At the end of the period, the alternator is reconnected to the regulator and standard load circuit. It is desirable to terminate the heating periods in some controlled fashion so that the window material is not excessively heated, which may cause damage, and so that the battery is not overly utilized to supply the system load requirements. A well known manner of controlling the duration of the heating period is to sense the window surface temperature, and to terminate the supply of heat upon a certain temperature being reached. Such form of control requires the use of temperature sensor elements that are either embedded within the window material or attached to the window. These systems are costly and undesirable for a number of reasons. The need for added electrical connections to the window, and the sophisticated fabrication techniques required for embedded sensor elements are distinct limitations. In addition, both the embedded and attached sensors must provide a distributed sensing function for sensing temperature throughout the area of the window surface, and must be mounted so as to not impair good vision.